Abstract

The growth rate of climate forcing by measured greenhouse gases
peaked near 1980 at almost 5 W/m2 per century.
This growth rate has since declined to ≈3 W/m2 per
century, largely because of cooperative international actions. We argue
that trends can be reduced to the level needed for the moderate
“alternative” climate scenario (≈2 W/m2 per
century for the next 50 years) by means of concerted actions that have
other benefits, but the forcing reductions are not automatic
“co-benefits” of actions that slow CO2 emissions.
Current trends of climate forcings by aerosols remain very uncertain.
Nevertheless, practical constraints on changes in emission levels
suggest that global warming at a rate +0.15 ± 0.05°C per decade
will occur over the next several decades.

Global surface temperature
increased in the past century by more than 0.5°C (1, 2). This warming
is, at least in part, a result of anthropogenic climate forcing agents
(3).

A climate forcing is an imposed, natural, or anthropogenic perturbation
of the Earth's energy balance with space (3, 4). Increasing
anthropogenic greenhouse gases (GHGs) cause the largest positive
(warming) forcing. Thus the proposed Kyoto Protocol
(http://www.unfccc.org/resource/docs/convkp/kpeng.html)
is designed to slow emissions of several GHGs.

We calculate the trend of climate forcing caused by measured changes in
GHGs and discuss implications. We emphasize the importance of other
forcings that are not well measured.

Climate Forcings

Fig. 1 shows estimates of climate
forcings since 1850. Most of these are similar to previous estimates
(3, 5), but a few key forcings warrant discussion.

Methane.

The estimated CH4 forcing is half as large as
that for CO2. CH4 is
included in the Kyoto Protocol but with a small weight compared with
CO2. We will argue that CH4
deserves greater attention and should not be lumped together with
CO2.

The CH4 forcing of 0.7
W/m2, composed of 0.5
W/m2 direct forcing and 0.2
W/m2 indirect forcing, is based on
line-by-line radiation calculations using current absorption line data
(5). Recent estimates of the direct forcing based in part on satellite
measurements of CH4 (6, 7) are slightly larger
than our value, whereas the Intergovernmental Panel on Climate Change
(IPCC) estimate of 0.49 W/m2 for
1750–2000 (3) is slightly smaller.

Our calculated indirect forcing of 0.1
W/m2 for CH4
oxidized to H2O in the stratosphere (5), based on
a chemical transport model (8), requires only one-third of the observed
stratospheric H2O increase (9) to be ascribed to
CH4. The indirect forcing of 0.1
W/m2 for the effect of increasing
CH4 on O3 (5) requires
one-eighth to one-quarter of the tropospheric O3
increase to be ascribed to CH4.

Troposphere Ozone.

Recent research (ref. 10; D. Shindell, personal communication) suggests
that the O3 forcing could be 0.7–0.8
W/m2, rather than the 0.3–0.4
W/m2 commonly assumed (3). Limited
observations of ozone in the 19th century imply the larger forcing, but
calibration of that data is uncertain and atmospheric chemistry models
did not yield such a large change. However, Mickley et al.
(10) show that uncertainties in the magnitude of natural ozone
precursors, especially NOX from lightning and
soils, are enough to permit the larger ozone change. We conclude that
the O3 forcing is probably in the range 0.4 to
0.8 W/m2.

Black Carbon (BC).

BC aerosols (soot), formed by incomplete combustion, cause a positive
climate forcing by absorbing sunlight and heating the lower atmosphere
(12). IPCC (3) estimates the BC forcing as +0.25
W/m2, but it is very uncertain. Jacobson
(13) calculates a BC forcing of ≈0.5 W/m2, including
enhanced absorption that occurs with internal mixing of aerosols.

Hansen (14) suggests that indirect effects of BC may increase its net
forcing to 0.5–1 W/m2. A decrease in the
mean albedo of snow and sea ice by 0.02, for example, would cause a
forcing of 0.25 W/m2. Limited data (15,
16) for the effect of BC on snow and sea ice albedo in the Northern
Hemisphere suggest a forcing at least that large. Perhaps this effect
contributes to observed sea ice loss in the Northern Hemisphere (17).

Observations of BC aerosols are inadequate to define their forcing.
However, from a survey of observations we estimate the mean single
scatter albedo of anthropogenic aerosols, weighted by optical depth, as
≈0.95. The aerosol absorption, mainly caused by BC, reduces the
negative aerosol forcing by sulfates, nitrates, and organic carbon by
about half (4). Thus the reflective aerosol forcing (Fig. 1)
implies a direct BC forcing of ≈0.6 W/m2
and supports inference of a net BC forcing of 0.5–1
W/m2.

Reflective Aerosols.

Estimates of climate forcing by sulfate aerosols fall mainly in the
range −0.3 to −1.0 W/m2 (3). However,
the smaller values do not fully account for the swelling of sulfate
aerosols in regions of high humidity (18). Thus the sulfate forcing
probably falls in the range −0.6 to −1.0
W/m2.

Organic aerosols come from biomass burning, fossil fuels, and
atmospheric oxidation of biogenic and anthropogenic volatile organic
compounds. The estimated anthropogenic forcing, −0.3
W/m2, is uncertain by about a factor of 2
(3).

Ammonium nitrate is estimated to cause a climate forcing of about −0.2
W/m2 (18). Nitrates are not included in
the IPCC estimates of climate forcing (3). The magnitude of the forcing
estimated here (Fig. 1) for reflective aerosols and soil dust
(−1.4 ± 0.5 W/m2) is about double
the central IPCC estimate, but it is within their range of uncertainty
(chapter 5 in ref. 3).

Net Climate Forcing.

Other climate forcings in Fig. 1 are discussed elsewhere (3, 5). The
sum of all of the positive forcings is 4.3 ± 0.6
W/m2, about three times greater than the
CO2 forcing (1.4 ± 0.2
W/m2). The sum of the negative forcings is
−2.7 ± 0.9 W/m2, and the net
forcing is 1.6 ± 1.1 W/m2.

Most climate simulations, as summarized by the IPCC (3), do not include
all of the negative forcings of Fig. 1; indeed, if they did, and other
forcings were unchanged, little global warming would be obtained.
However, they also do not include the full BC,
O3, and CH4 forcings,
estimated in Fig. 1 as 2 W/m2. With this
forcing included, and a climate sensitivity of ¾°C per
W/m2, realistic rates of warming and heat
storage in the ocean are obtained (8, 19). Although the sum of all
forcings coincidentally is similar to that for
CO2 alone, knowledge of each of the large
forcings in Fig. 1 is needed for development of effective policies.

Greenhouse Gas Trends

Fig. 2A shows the growth
rate of atmospheric CO2. Data before 1958 are
obtained from bubbles of air trapped in polar ice sheets, whereas
subsequent data are atmospheric observations.

Growth rate of atmospheric CO2 and CH4 based on
ice core measurements of Etheridge et al. (20, 21),
in situ CO2 observations initiated by
C. D. Keeling (11), and National Oceanic and Atmospheric
Administration (NOAA) CO2 and CH4 observations
made available by the NOAA Climate Monitoring and Diagnostics
Laboratory (ftp://ftp.cmdl.noaa.gov/ccg).

The annual growth of CO2 increased rapidly
between World War II and the oil crisis of the mid-1970s, an interval
during which fossil fuel CO2 emissions increased
exponentially at more than 4%/year. Since then the annual
CO2 growth has been relatively flat at about 1.5
ppm/year. As fossil fuel CO2 emissions
have increased at about 1%/year during the past 25 years (see
below), the flat growth rate implies some increase in the net
terrestrial and/or oceanic uptakes of CO2.

Fig. 2B shows the growth rate of atmospheric
CH4. After World War II the annual growth of
CH4 increased from about 5 ppb/year to
about 15 ppb/year. Even though data are limited for 1965–1985,
it is clear, based on the prior and subsequent absolute
CH4 amounts, that growth averaged about 15
ppb/year during that period. Since 1980 the
CH4 growth rate has declined by about two-thirds.
Causes of the slower CH4 growth are uncertain, as
both a leveling off of CH4 sources (21, 22) and a
decreasing CH4 lifetime (23–25) may contribute.
Better measurements are needed of the trends of
CH4 sources and emissions that affect atmospheric
OH, the primary sink for CH4.

Fig. 3 shows the growth rate for climate
forcing by several chlorofluorocarbons, hydrochlorofluorocarbons,
chlorocarbons, and bromocarbons, which we abbreviate together as CFCs.
These 13 gases are being phased out by the Montreal Protocol because of
their destructive effect on stratospheric ozone. In the 1970s and 1980s
their forcing increased at a rate of more than 1
W/m2 per century. As the emission of these
gases has slowed, the growth rate of their climate forcing has fallen
dramatically.

The red line in Fig. 3 is the history and projection of the 11 CFCs
other than CFC-11 and CFC-12 (appendix II of ref. 3). Recent
observations are available for eight of the 11 gases (26). The 11-gas
forcing growth rate using observations (Fig.
4) is below the IPCC estimate, and it is
less in 2000 than in 1999. This finding suggests that the 13-gas CFC
forcing growth rate may turn negative sooner than the 2010 date of the
IPCC projections.

Growth rate of climate forcing by well-mixed greenhouse gases (5-year
mean, except 3-year mean for 1999 and 1-year mean for 2000).
O3 and stratospheric H2O, which were not well
measured, are not included.

Greenhouse Gas Climate Forcing Trend

Fig. 4 shows the growth rate of GHG forcing, which peaked about
1980 at almost 5 W/m2 per century. If that
rate had been maintained, a forcing equivalent to doubled
CO2 (4 W/m2) would
have been obtained by 2050. Some climate scenarios in the 1980s assumed
that the exponential growth of 1945–1975 would continue, leading to
doubled CO2 forcing by 2025 (14). In reality, the
growth rate has decelerated to about 3
W/m2 per century.

The slowdown was caused mainly by the Montreal Protocol phase-out of
ozone-depleting gases. The protocol has been a model of international
environmental cooperation as developed countries produced alternative
technologies and provided a multilateral fund to help developing
countries replace CFC technologies. The cost over a decade was about $1
billion.

Another factor allowing slowdown of the climate forcing growth rate was
flattening of the CO2 growth rate (Fig.
2A), which was related to the slower growth rate of
fossil fuel CO2 emissions (Fig.
5), discussed above in connection with
Fig. 2A.

The recent flat growth rate of CO2, despite
continuing increase of CO2 emissions at about
1%/year, is in part a reflection of increased terrestrial
sequestration of carbon in the 1990s (28). A flat growth rate of the
CO2 forcing probably can be maintained only by
means of further slowing of CO2 emissions growth
to about 0%/year, i.e., constant emissions. Stabilization of
atmospheric composition will require that CO2
emissions eventually be reduced by 50–85% (3), unless technology for
CO2 sequestration is developed. We refer here to
geological sequestration or injection into the deep ocean.

Fig. 6 shows the United States portion of
global fossil fuel CO2 emissions, which increased
from 10% in 1850 to 50% in 1920 as the U.S. grew and industrialized.
The U.S. portion has since declined to 23% as the rest of the world
industrialized, but there was a temporary spike back to 50% at the end
of World War II as U.S. industry supplied the war effort.

The principal relevance of Fig. 6 is the flattening of the trend of the
U.S. portion of CO2 emissions during the 1990s,
despite increasing awareness of the climate issue. As the U.S. is a
global leader in technology development, it seems unlikely that the
global CO2 emissions curve (Fig. 5) will level
off and eventually decline unless the U.S. aggressively develops energy
efficiency and non-CO2 energy sources, or
CO2 sequestration.

Global Warming Potentials

Fig. 1 indicates that several climate forcing agents are
significant. Thus it is important to compare the effectiveness of
reducing the emissions of each of these constituents. For that purpose
IPCC (3) uses global warming potentials (GWPs).

IPCC defines GWPs relative to CO2 and weights
them by constituent lifetime. The detour of GWP through properties of
CO2 makes it unnecessarily difficult to relate a
change in emissions to expected global warming. The Kyoto Protocol
chooses the 100-year GWP, which makes it appear that short-lived
constituents have little value for slowing global warming.

We propose an alternative GWP (GWPa).
GWPa is simply the change in climate forcing
caused by a change in emissions. This forcing can be converted to an
expected long-term global temperature change by multiplying by climate
sensitivity, which is estimated from empirical evidence and climate
models to be ¾ ± ¼°C per
W/m2 (29).

We choose 50 years as the principal time frame. One hundred years is
too long, because we cannot discern technology changes that will occur
by 2100. A time frame less than several decades is inappropriate,
because of the long life of energy infrastructure.

The present climate forcing by each constituent enters as a simple
product (Table 1). Thus the forcing
change as a function of emission change can be adjusted as knowledge of
the present forcing improves. We use a BC forcing (0.6
W/m2) smaller than in Fig. 1 and closer to
other estimates (13), thus strengthening conclusions below. The sulfate
forcing includes little or no aerosol indirect forcing and is
especially uncertain.

We illustrate GWPa by considering plausible
changes in the climate forcing agents and examining whether these would
be sufficient to achieve the “alternative scenario” that we
proposed previously (5). That scenario aims to keep the added climate
forcing in the next 50 years at 1 W/m2 or
less, and thus keep the global warming in that period at ¾°C
or less (5, 8).

Carbon Dioxide.

IPCC scenarios for CO2 are a product of
assumptions for population, living standards, energy sources, and
technology, resulting in a huge range of CO2
trends (3, 14). Our approach is to use the historical
CO2 emissions curve (Fig. 5), which includes
these factors. We expect the energy supply/cost factors that
caused the growth rate to decline from 4–4.5%/year to
1–1.5%/year to continue. We consider possible additional
downward pressure caused by climate concerns.

If CO2 continues to increase 1.5 ppm/year,
which would require that global emissions be kept about the same as
today, the added forcing in 50 years will be ≈1.08
W/m2 (5). Achievement of flat
CO2 emissions will require major efforts in
energy efficiency, fuel switching, and renewable energies. If, rather
than being constant, CO2 emissions increase
exponentially at 1.5%/year, the added forcing in 50 years is
1.54 W/m2. This growth rate is perhaps the
largest plausible one, exceeding that of the past 25 years. If
1.5%/year growth occurs in developing countries and emissions
in developed countries are constant at the 2000 level, the added
forcing is 1.28 W/m2 (Table 1).

Methane.

The IPCC estimate of additional CH4 forcing
between 2000 and 2050 declined to 0.04–0.31
W/m2 in their recent report (3) from
0.17–0.44 W/m2 in their previous report.
Methane growth in the 1990s fell well below all IPCC scenarios (5).

Our alternative scenario (5) aims for a 30% reduction of
CH4 forcing by means of actions such as improved
agricultural practices and capture of CH4
escaping from landfills, coal mining, oil and gas production, and
anaerobic waste management lagoons. The required reduction of
anthropogenic CH4 sources is ≈25%, because of
OH feedback (30). These actions have economic benefits that help offset
costs (31), but they are unlikely to occur without global cooperation
and sharing of technology.

A 30% reduction of the CH4 forcing yields a
negative forcing of −0.21 W/m2 (Table 1).
This diminution of climate forcing is comparable to that expected in
the next 50 years from the Kyoto Protocol, which aims to reduce GHG
emissions of developed countries by the equivalent of ≈5% of their
CO2 emissions.

Air Pollution.

IPCC scenarios (3) have air pollution, and thus climate forcing by
O3 and BC, increasing in the next 50 years.

We argue that human and economic costs of air pollution make a
global focus on air pollution desirable, if not inevitable (5).

Air pollution growth will slow as a co-benefit of slower fossil fuel
growth. However, we contend that absolute reductions of
O3 and BC are possible. BC is a product of
incomplete combustion, whereas O3 is produced by
emissions of reactive organic gases (ROGs), NOX,
CO, and CH4 (30). Improved combustion
technologies can reduce emissions of both BC and
O3 precursors (32). If anthropogenic
CH4 is reduced 30%, emissions of ROGs,
NOX, and CO must be reduced about 40% to achieve
a 30% reduction of anthropogenic O3 (ref. 30; M.
Prather, personal communication). We suggest that 30% reduction of
global O3 pollution by 2050 is a realistic
goal (5), based on reductions in some developed countries (32) and
technology trends.

BC (soot) emissions are sensitive to industrial and vehicular
technologies (33) and domestic cooking and heating technologies (34).
Our alternative scenario (5) aims to reduce anthropogenic BC emissions
enough to balance warming from reduced sulfates (Table 1). Sulfates,
the main source of acid rain, need to be reduced, so we suggest an
emphasis on also reducing BC.

Aerosol climate forcing will be very uncertain until global
composition-specific aerosol measurements allow definition of trends.
Our estimated forcing of +0.3 W/m2 for
reduction of reflective aerosols does not include changes of the
indirect aerosol effect, which is very uncertain. If the indirect
aerosol forcing is large, reduction of reflecting aerosols could
seriously aggravate global warming. Given that oceans cover 70% of the
world, one stop-gap strategy may be to allow ships to continue to use
high sulfur fuel for a time. However, we do not recommend use of one
pollutant to mask the effect of another. It may be possible to minimize
undesirable effects with optimum prioritization and sequencing of
emission reductions, but recommendations require better understanding
of aerosol climate forcings and their environmental effects.

Other Forcings.

Table 1 includes other climate forcing changes anticipated by 2050. The
N2O increase, based on current trends (8), is
similar to the IPCC (3) estimate. The CFC decrease, from Fig. 3, is in
accord with the Montreal Protocol and IPCC (3) estimates, as is the
resulting stratospheric O3 recovery.

Trace in Table 1 refers to trace gases outside the Montreal Protocol,
including perfluorocarbons, hydrofluorocarbons, and
SF6. IPCC (3) scenarios yield a forcing ranging
from 0.09 to 0.19 W/m2. The higher values
are caused mainly by HFC-134a, whose growth rate has fallen below IPCC
estimates. We assume an additional forcing of 0.1
W/m2 by these trace gases; however, it
could be reduced by minimizing HFC-134a production (5).

Net GWPa, 2000–2050.

The added climate forcing in the next 50 years is 0.85
W/m2 if CO2 growth
is flat at 1.5 ppm/year and if the above reductions of
CH4, O3, and BC are
achieved. Kyoto emission reductions do not appear explicitly in this
case, but note that keeping CO2 emissions flat
surely requires improved energy efficiencies, fuel switching, and
alternative energy sources as envisaged under Kyoto emission
restrictions.

If global CO2 emissions increase exponentially at
1.5%/year for the next 50 years, but air pollution and
CH4 emissions are reduced as suggested above, the
forcing increase is 1.31 W/m2. This
forcing scenario exceeds the goal of 1
W/m2 or less in the alternative scenario.
Moreover, the forcing growth rate is accelerating in 2050, contrary to
the goal of decelerating toward stabilization of atmospheric
composition later in the century.

Fig. 7 summarizes differences between the
alternative scenario and IPCC scenarios (3). First,
CH4, O3, and BC increase in
the IPCC scenarios, whereas the alternative scenario has a global
reduction of air pollution. Second, the IPCC includes
CO2 growth rates that we contend are
unrealistically large.

Climate forcing scenario for 2000–2050 that yields a forcing of 0.85
W/m2 (colored bars), including small forcings from
stratospheric ozone recovery and trace gases (Table 1).

Our approach for CO2 is to study historical
emission trends (Fig. 5) and consider plausible changes. Exponential
emissions growth at 1.5%/year for 50 years yields a
CO2 forcing of 1.54
W/m2 (Table 1). A forcing of 3
W/m2 requires more than 4%/year
exponential growth for 50 years. Such a scenario understates trends
toward improved energy efficiency and decarbonization of energy sources
(figure 8 in ref. 35). The factors that caused the growth rate to slow
from 4%/year to 1%/year since the 1970s remain in place
and are joined by a slowing population growth rate (36) and
international concerns about global warming. Reacceleration from
1%/year to 4%/year growth does not seem credible.

Global Warming, 2000–2050.

A byproduct of the above analysis is the conclusion that future global
warming can be predicted much more accurately than is generally
realized. We show elsewhere (8) that a forcing of 1.08
W/m2 yields a warming of ¾°C by
2050 in transient climate simulations with a model having equilibrium
sensitivity of ¾°C per W/m2.

We contend that a forcing much smaller than 0.85
W/m2 is unlikely, because fossil fuels are
expected to be the primary energy source for at least several decades.
Rapid introduction of nonfossil energies or CO2
sequestration might reduce the forcing by a few tenths of 1
W/m2. However, much of the warming in the
next 50 years will be from presently “unrealized warming” caused
by the existing planetary radiative imbalance of at least 0.5
W/m2 (8, 37). Slowing
CO2 emissions in the second quartile of the
century, although crucial for stabilizing atmospheric composition later
in the century, would have only a small effect on the warming in 2050.
These considerations suggest a minimum warming of 0.5°C by
2050.

At the other extreme, CO2 growth exceeding
exponential at 1.5%/year would be inconsistent with historical
trends and with the negative feedback caused by human concern about
climate change. Thus the maximum CO2 forcing is
1.28–1.54 W/m2 (Table 1). BC and
O3 are unlikely to be much greater in 2050 than
today. Indeed, China has already begun to reduce its air pollution (38)
and other developing countries are probably near their limits.
Continued global warming would produce at least moderate public
concern, thus limiting added forcing to about 1.5
W/m2 and realized warming to about 1°C.

Given these constraints on climate forcing trends, we predict
additional warming in the next 50 years of ¾ ±
¼°C, a warming rate of 0.15 ± 0.05°C per decade. A
slower warming rate will occur in the second half of the century,
assuming that the climate forcing growth rate begins to trend downward
before 2050.

Summary

A remarkable deceleration in the growth rate of GHG climate
forcing occurred in the past 20 years. The slowdown was caused mainly
by phase-out of CFCs. It was accomplished by means of cooperative, not
punitive, international actions. Developed countries, through the
Global Environmental Facility of the World Bank, provided support to
developing countries for alternative technologies and phase-out of CFC
production. Similar cooperation on other climate forcings could
alleviate future global warming.

Methane.

CH4 has analogies to CFCs. Technologies are
within reach for reducing CH4 emissions. As with
CFCs, the cost of actions to reduce CH4 can be
much less than the cost of dealing with CO2.
Developed countries, in addition to reducing their own sources, could
support the implementation of required technologies in developing
countries. By targeting emission reductions that have some economic
benefits, it should be possible to find self-sustaining reductions.

Methane, with a climate forcing half as great as that of
CO2, provides an opportunity for a global warming
success story. A halt and even reversal of its growth is possible, it
could occur quickly, and it could provide an example for cooperation on
CO2. Success in halting or even reversing growth
of CH4 will not remove the need to slow
CO2 emissions.

Air Pollution.

Ozone and aerosols are prime air pollutants. All countries have strong
incentives for reducing air pollution, which affects human health and
agricultural productivity. In India alone it is estimated that 270,000
children under age 5 die each year of acute respiratory infections
caused by particulate air pollution (39). Global deaths from air
pollution are at least one million annually (40). Economic costs
in several European countries are estimated at 1.6% of their
gross domestic products (41).

Climate forcing by O3 and BC aerosols combined
may be comparable to that of CO2 (Fig. 1), yet
neither constituent is included in the Kyoto Protocol. Some reduction
in air pollution may occur as an incidental cobenefit of reduced fossil
fuel use, but the small Kyoto emission reduction may be unnoticeable.
Much larger reductions of air pollution are feasible. Achievement
requires a strategy that explicitly targets O3
and BC. Here again there is a need and opportunity for global
cooperation in technology development and implementation.

It should not be imagined that O3 and BC
reductions can halt global warming. Our aim in targeting
O3 and BC is to alleviate the warming expected to
accompany reduction of reflective aerosols. This scenario contrasts
with the IPCC scenarios, which have increasing O3
and BC in the next 50 years (Fig. 7). Targeting
O3 and BC moderates the warming while delivering
great benefits to human health and agricultural productivity. However,
it does not remove the need to slow CO2
emissions.

Carbon Dioxide.

Increasing CO2 causes the largest positive
climate forcing now and is likely to be the dominant forcing in the
future. Added CO2 forcing in the next 50 years
should be about 1 W/m2 if
CO2 emissions level out at today's amount. This
level is far less than in business-as-usual scenarios that yield a
specter of imminent disaster, but it is enough to cause substantial
climate change. If flat emissions continued indefinitely, warming would
be expected to continue at a rate 1.5 ± 0.5°C per century. Thus
CO2 emissions must be curtailed eventually, or
captured and sequestered, to stabilize atmospheric composition.

A reasonable immediate goal is flat global emissions in the present
decade by emphasizing energy efficiency, fuel switching, and renewable
energy. This goal seems feasible, as the growth in
CO2 emissions in the past decade was less than
1%/year, despite strong global economic growth. However,
achievement of flat emissions is unlikely without concerted actions to
remove barriers to efficiency (42) and promote
non-CO2 energy sources.

As energy needs grow, it will be necessary to have increasing amounts
of energy from sources producing little CO2 or to
capture and sequester fossil fuel CO2. Capture of
CO2 becomes more practical as an increasing
portion of energy use is in the form of clean electrical energy
generated at power stations.

Competitive development of technologies that produce little
CO2, or sequester it, should be accelerated now.
Then, as evidence for and concern about climate change increases, it
may become feasible to achieve a slowing of CO2
emissions and a forcing growth rate even smaller than 1
W/m2 in 50 years.

Global Warming.

Current trends and projections of climate forcings lead us to predict
global warming for several decades at a rate 0.15 ± 0.05°C per
decade. Although this warming is more moderate than in
business-as-usual scenarios, if it is maintained for a century the
Earth's temperature will approach that of the middle Pliocene (2.75
million years ago), when the world was about 2°C warmer than today
and sea level was at least 25 m higher (43). This conclusion supports
the need for actions that slow the growth of climate forcings.

It is now impossible to avoid global warming this century. However, the
actions outlined here can slow the warming, while having other benefits
that justify the actions. If CO2 emissions are
kept level, and if technology is developed to reduce or capture
emissions in the second quartile of the century, it should be possible
to limit midcentury warming to 0.5°C and stabilize atmospheric
composition later in the century.

Improved measurements of all climate forcings are needed to design
optimum policies, which must be adjusted as understanding develops.
Climate change is a long-term problem.

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